U.S. patent application number 09/207137 was filed with the patent office on 2001-09-27 for image forming apparatus.
Invention is credited to KIMIZUKA, JUNICHI, TOYOIZUMI, KIYOTO.
Application Number | 20010024308 09/207137 |
Document ID | / |
Family ID | 18330959 |
Filed Date | 2001-09-27 |
United States Patent
Application |
20010024308 |
Kind Code |
A1 |
TOYOIZUMI, KIYOTO ; et
al. |
September 27, 2001 |
IMAGE FORMING APPARATUS
Abstract
In an image forming apparatus, a plurality of laser beams are
emitted by a plurality of laser sources and are scanned onto a
recording medium. The laser beams scanned are detected by two
light-receiving elements juxtaposed in a main scanning direction.
The output signals from the two light-receiving elements are
compared so that a reference timing signal indicating that each of
the scanned laser beams has reached a substantially middle position
between the two light-receiving elements is outputted. The write
operations of the plurality of laser sources in the main scanning
direction are controlled to be synchronized on the basis of the
respective reference timing signals.
Inventors: |
TOYOIZUMI, KIYOTO;
(KANAGAWA-KEN, JP) ; KIMIZUKA, JUNICHI;
(KANAGAWA-KEN, JP) |
Correspondence
Address: |
FITZPATRICK CELLA HARPER & SCINTO
30 ROCKEFELLER PLAZA
NEW YORK
NY
10112
US
|
Family ID: |
18330959 |
Appl. No.: |
09/207137 |
Filed: |
December 8, 1998 |
Current U.S.
Class: |
359/204.1 |
Current CPC
Class: |
H04N 2201/04712
20130101; H04N 1/1135 20130101; H04N 2201/04744 20130101; H04N 1/12
20130101; H04N 1/053 20130101; H04N 2201/04732 20130101; H04N
1/1911 20130101; H04N 2201/04786 20130101 |
Class at
Publication: |
359/204 |
International
Class: |
G02B 026/08 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 10, 1997 |
JP |
09-339802 |
Claims
What is claimed is:
1. An image forming apparatus comprising: a plurality of laser
sources; scanning means for scanning laser beams emitted by said
plurality of laser sources onto a recording medium; two
light-receiving elements juxtaposed in a main scanning direction to
detect the laser beams scanned by said scanning means; means for
comparing output signals from said two light-receiving elements,
and outputting a reference timing signal indicating that each of
the scanned laser beams has reached a substantially middle position
between said two light-receiving elements; and control means for
synchronizing write operations of said plurality of laser sources
in the main scanning direction on the basis of the reference timing
signal.
2. An apparatus according to claim 1, wherein said two
light-receiving elements detect the laser beams scanned by said
scanning means prior to scanning on the recording medium in the
main scanning direction.
3. An apparatus according to claim 1, wherein when the laser beams
scanned by said scanning means pass surfaces of said two light
receiving elements, said plurality of laser sources are turned on
in an order from a leading laser beam.
4. An apparatus according to claim 1, wherein a downstream one of
said two light-receiving elements in the main scanning direction
has a width smaller than an upstream light-receiving element.
5. An apparatus according to claim 1, wherein said two
light-receiving elements comprise a 2-split photodetection element
split in the main scanning direction.
6. An apparatus according to claim 1, wherein when each of the
laser beams scanned by said scanning means has exceeded the
substantially middle position between said two light-receiving
elements, the scanned laser beam is turned off.
7. An apparatus according to claim 1, wherein said two
light-receiving elements include upstream and downstream
light-receiving elements, and when each of the laser beams scanned
by said scanning means falls substantially outside the upstream
light-receiving element, the scanned laser beam is turned off.
8. A scanning optical device comprising: a plurality of laser
sources; scanning means for scanning laser beams emitted by said
plurality of laser sources; two light-receiving elements juxtaposed
in a main scanning direction to detect the laser beams scanned by
said scanning means; means for comparing output signals from said
two light-receiving elements, and outputting a reference timing
signal indicating that each of the scanned laser beams has reached
a substantially middle position between said two light-receiving
elements; and control means for synchronizing write operations of
said plurality of laser sources in the main scanning direction on
the basis of the reference timing signal.
9. A device according to claim 8, wherein said two light-receiving
elements detect the laser beams scanned by said scanning means
prior to scanning on the recording medium in the main scanning
direction.
10. A device according to claim 8, wherein when the laser beams
scanned by said scanning means pass surfaces of said two light
receiving elements, said plurality of laser sources are turned on
in an order from a leading laser beam.
11. A device according to claim 8, wherein a downstream one of said
two light-receiving elements in the main scanning direction has a
width smaller than an upstream light-receiving element.
12. A device according to claim 8, wherein said two light-receiving
elements comprise a 2-split photodetection element split in the
main scanning direction.
13. A device according to claim 8, wherein when each of the laser
beams scanned by said scanning means has exceeded the substantially
middle position between said two light-receiving elements, the
scanned laser beam is turned off.
14. A device according to claim 8, wherein said two light-receiving
elements include upstream and downstream light-receiving elements,
and when each of the laser beams scanned by said scanning means
falls substantially outside the upstream light-receiving element,
the scanned laser beam is turned off.
15. A method of detecting a write start position in a main scanning
direction in an image forming apparatus having a plurality of laser
sources, comprising the steps of: scanning laser beams emitted by
said plurality of laser sources; detecting the scanned laser beams
scanned by two light-receiving elements juxtaposed in the main
scanning direction; comparing output signals from said two
light-receiving elements, and outputting a reference timing signal
indicating that each of the scanned laser beams has reached a
substantially middle position between said two light-receiving
elements; and synchronizing write operations of said plurality of
laser sources in the main scanning direction on the basis of the
reference timing signal.
16. A method according to claim 15, wherein said two
light-receiving elements detect the laser beams scanned by said
scanning means prior to scanning on the recording medium in the
main scanning direction.
17. A method according to claim 15, wherein when the laser beams
scanned by said scanning means pass surfaces of said two light
receiving elements, said plurality of laser sources are turned on
in an order from a leading laser beam.
18. A method according to claim 15, wherein a downstream one of
said two light-receiving elements in the main scanning direction
has a width smaller than an upstream light-receiving element.
19. A method according to claim 15, wherein said two
light-receiving elements comprise a 2-split photodetection element
split in the main scanning direction.
20. A method according to claim 15, wherein when each of the laser
beams scanned by said scanning means has exceeded the substantially
middle position between said two light-receiving elements, the
scanned laser beam is turned off.
21. A method according to claim 15, wherein said two
light-receiving elements include upstream and downstream
light-receiving elements, and when each of the laser beams scanned
by said scanning means falls substantially outside the upstream
light-receiving element, the scanned laser beam is turned off.
22. An image forming apparatus comprising: a plurality of laser
sources; scanning means for scanning laser beams emitted by said
plurality of laser sources onto a recording medium; two
light-receiving elements juxtaposed in a main scanning direction to
detect the laser beams scanned by said scanning means; means for
comparing output signals from said two light-receiving elements,
and outputting, for said plurality of laser sources, reference
timing signals each indicating that each of the scanned laser beams
has reached a substantially middle position between said two
light-receiving elements; and control means for synchronizing write
operations of said plurality of laser sources in the main scanning
direction on the basis of the reference timing signals respectively
outputted for said plurality of laser sources.
23. A scanning optical device comprising: a plurality of laser
sources; scanning means for scanning laser beams emitted by said
plurality of laser sources; two light-receiving elements juxtaposed
in a main scanning direction to detect the laser beams scanned by
said scanning means; means for comparing output signals from said
two light-receiving elements, and outputting, for said plurality of
laser sources, reference timing signals each indicating that each
of the scanned laser beams has reached a substantially middle
position between said two light-receiving elements; and control
means for synchronizing write operations of said plurality of laser
sources in the main scanning direction on the basis of the
reference timing signals respectively outputted for said plurality
of laser source.
24. A method of detecting a write start position in a main scanning
direction in an image forming apparatus having a plurality of laser
sources, comprising the steps of: scanning laser beams emitted by
said plurality of laser sources; detecting the scanned laser beams
scanned by two light-receiving elements juxtaposed in the main
scanning direction; comparing output signals from said two
light-receiving elements, and outputting, for said plurality of
laser sources, reference timing signals each indicating that each
of the scanned laser beams has reached a substantially middle
position between said two light-receiving elements; and
synchronizing write operations of said plurality of laser sources
in the main scanning direction on the basis of the reference timing
signals respectively outputted for said plurality of laser sources.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a multi-beam write image
forming apparatus for optically writing using a plurality of light
beams.
[0003] 2. Related Background Art
[0004] In a conventional a multi-beam write scheme, a plurality of
semiconductor lasers are used, and laser beams emitted by these
semiconductor lasers are simultaneously scanned in the main
scanning direction on a photosensitive body to simultaneously write
a plurality of lines. It is important in such write scheme to align
the start points of the individual beams in the main scanning
direction so as to obtain a high-precision recorded image. In a
conventional system, for example, two laser beams reflected by a
rotary polygonal mirror 100 are split by a polarization beam
splitter 101, and the split laser beams are respectively detected
by photodetection elements 102 and 103, respectively. The signals
output from these two photodetection elements 102 and 103 are used
as reference timings of the individual laser beams, and a write
starts on a photosensitive body a predetermined period of time
after the photodetection elements 102 and 103 detect the laser
beams.
[0005] However, in such detection method, a plurality of optical
elements such as a polarization beam splitter, photodetection
elements, and the like are required, and the apparatus arrangement
is complicated. Also, depending on the setup angle of the
polarization beam splitter and mounting precision of the
photodetection elements, the time from the detection of the two
laser beams to the write start positions changes. For this reason,
upon mounting these polarization beam splitter and photodetection
elements, strict mounting precision is required, resulting in
time-consuming mounting.
[0006] Also, since the amount of light that becomes incident on
each photodetection element changes due to variations in
reflectance and contaminations of the respective reflection
surfaces of the rotary polygonal mirror, the time required until
the threshold value of each photodetection element is exceeded
changes, and the generation timing of the output signal of each
photodetection element also changes. However, when the timing of
the output signal of the photodetection element has changed, since
the reference timings of two lines are determined by the output
signals from the respective photodetection elements, the write
start positions of the two laser beams in the main scanning
direction deviate, and the start points in the main scanning
direction cannot be accurately aligned.
SUMMARY OF THE INVENTION
[0007] The present invention has been made in consideration of the
conventional problems, and has as its object to provide an image
forming apparatus which can obtain a high-precision recorded image
free from any deviation of a plurality of write lines with a simple
arrangement.
[0008] The object of the present invention is achieved by an image
forming apparatus, which comprises a plurality of laser sources and
writes a plurality of lines on an image carrier by simultaneously
scanning laser beams emitted by the respective laser sources in the
main scanning direction, characterized by comprising two
light-receiving elements juxtaposed in the main scanning direction
to detect the laser beams emitted by the plurality of laser sources
prior to scanning in the main scanning direction, means for turning
on spots formed by the laser beams from the plurality of laser
sources in the order from a leading spot, and means for comparing
output signals from the two light-receiving elements, and
outputting a reference timing signal indicating that each spot has
reached a reference position when the spot is located at
substantially the middle position between the two light-receiving
elements, and in that write operations of the plurality of laser
sources in the main scanning direction are synchronized on the
basis of the output reference timing signal of the spots.
[0009] An image forming apparatus according to the present
invention is characterized by comprising:
[0010] a plurality of laser sources;
[0011] scanning means for scanning laser beams emitted by the
plurality of laser sources onto a recording medium;
[0012] two light-receiving elements juxtaposed in a main scanning
direction to detect the laser beams scanned by the scanning
means;
[0013] means for comparing output signals from the two
light-receiving elements, and outputting a reference timing signal
indicating that each of the scanned laser beams has reached a
substantially middle position between the two light-receiving
elements; and
[0014] control means for synchronizing write operations of the
plurality of laser sources in the main scanning direction on the
basis of the reference timing signal.
[0015] The image forming apparatus according to the present
invention is characterized in that the two light-receiving elements
detect the laser beams scanned by the scanning means prior to
scanning on the recording medium in the main scanning
direction.
[0016] The image forming apparatus according to the present
invention is characterized in that when the laser beams scanned by
the scanning means pass surfaces of the two light receiving
elements, the plurality of laser sources are turned on in an order
from a leading laser beam.
[0017] The image forming apparatus according to the present
invention is characterized in that a downstream one of the two
light-receiving elements in the main scanning direction has a width
smaller than an upstream light-receiving element.
[0018] The image forming apparatus according to the present
invention is characterized in that the two light-receiving elements
comprise a 2-split photodetection element split in the main
scanning direction.
[0019] The image forming apparatus according to the present
invention is characterized in that when each of the laser beams
scanned by the scanning means has exceeded the substantially middle
position between the two light-receiving elements, the scanned
laser beam is turned off.
[0020] The image forming apparatus according to the present
invention is characterized in that the two light-receiving elements
include upstream and downstream light-receiving elements, and
[0021] when each of the laser beams scanned by the scanning means
falls substantially outside the upstream light-receiving element,
the scanned laser beam is turned off.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a view showing a conventional image forming
apparatus;
[0023] FIG. 2 is a sectional view showing the overall arrangement
of an image forming apparatus according to the first embodiment of
the present invention;
[0024] FIG. 3 is a plan view showing the arrangement of a scanner
unit of the image forming apparatus according to the first
embodiment of the present invention;
[0025] FIG. 4 is a view showing how two write spots of the
embodiment shown in FIG. 3 pass on two photodetection elements
juxtaposed in the main scanning direction;
[0026] FIG. 5 is a chart showing signals of the respective unit
when the two spots pass through the two photodetection
elements;
[0027] FIG. 6 is a circuit diagram showing an example of comparison
circuits used in the embodiment shown in FIG. 3;
[0028] FIG. 7 is a block diagram showing a control circuit for
ON/OFF-controlling the two spots when they pass through the two
photodetection elements;
[0029] FIG. 8 is a circuit diagram showing an example of a
separation circuit used in the embodiment shown in FIG. 3;
[0030] FIG. 9 is a chart showing signal separation of the
separation circuit shown in FIG. 8;
[0031] FIG. 10 is a chart showing input/output signals of a
waveform shaping circuit shown in FIG. 7;
[0032] FIG. 11 is a diagram and chart showing input/output signals
of a clock synchronous circuit (M66235FP available from Mitsubishi
Electric Corp.);
[0033] FIG. 12 is a chart showing signals of the respective units
in the embodiment shown in FIG. 3;
[0034] FIG. 13 is a view showing the conventional detection process
using a single photodetection element;
[0035] FIG. 14 is a chart showing a case in which the laser beam
amounts are different in the embodiment shown in FIG. 3;
[0036] FIG. 15 is a view for explaining another embodiment of the
present invention; and
[0037] FIG. 16 is a chart showing signals of respective units in
the embodiment shown in FIG. 15.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0038] FIG. 2 is a sectional view for explaining the overall
arrangement of an image forming apparatus according to the first
embodiment of the present invention, and exemplifies a laser
printer. The arrangement and operation of the apparatus will be
explained below.
[0039] A laser printer main body 101 (to be referred to as a main
body 101 hereinafter) has a cassette 102 for storing recording
sheets S, and a cassette sheet sensor 103 for detecting the
presence/absence of recording sheets S in the cassette 102, a
cassette size sensor 104 (constituted by a plurality of
microswitches) for detecting the size of recording sheets S in the
cassette 102, a pickup roller 105 for picking up a recording sheet
S from the cassette 102, and the like are placed around the
cassette 102. A pair of registration rollers 106 for synchronously
conveying a recording sheet S are located on the downstream of the
pickup roller 105. An image forming unit 108 for forming a toner
image on the recording sheet S on the basis of laser beams coming
from a laser scanner unit 107 is placed downstream the pair of
registration rollers 106.
[0040] Furthermore, a fixing device 109 for thermally fixing the
toner image formed on the recording sheet S is located downstream
the image forming unit 108, and an exhaust sensor 110 for detecting
the sheet convey state on an exhaust unit, exhaust rollers 111 for
exhausting the recording sheet S, and a tray 112 for stacking
recorded recording sheets S are placed downstream the fixing device
109.
[0041] The scanner unit 107 comprises a laser unit 100 for emitting
a laser beam modulated on the basis of an image signal (image
signal VDO) output from an external apparatus 128 (to be described
later), a rotary polygonal mirror 4 for scanning the laser beam
emitted by the laser unit 100 onto a photosensitive drum 6 (to be
described later), a polygon motor 114, an imaging lens group 5, a
return mirror 9, and the like. The image forming unit 108 comprises
the photosensitive drum 6, a pre-exposure lamp 118, a primary
charger 119, a developer 120, a transfer charger 121, a cleaner
122, and the like, which are required for the known
electrophotography process. Also, the fixing device 109 comprises a
heat roller 109a, compression roller 109b, halogen heater 109c
included in the heat roller, and thermistor 109d for detecting the
surface temperature of the heat roller.
[0042] A main motor 123 supplies a driving force to the pickup
roller 105 via a pickup roller clutch 124 and to the pair of
registration rollers 106 via a registration roller clutch 125, and
also supplies a driving force to the respective units of the image
forming unit 108 including the photosensitive drum 6, the fixing
device 109, and the exhaust rollers 111. A printer controller 126
controls the main body 101, and is constructed by an MPU
(microcomputer) 126d comprising a timer 126a, ROM 126b, RAM 126c,
and the like, various I/O control circuits (not shown), and the
like. Furthermore, the printer controller 126 is capable of
communicating with an external apparatus 128 via an interface
127.
[0043] FIG. 3 is a plan view showing the arrangement of the scanner
unit of the image forming apparatus according to the first
embodiment of the present invention. Referring to FIG. 3,
semiconductor lasers 1a and 1b serve as write light sources. The
semiconductor lasers 1a and 1b are driven by a laser drive circuit
(not shown) in accordance with an image signal, and emit laser
beams which are modulated in accordance with the image signal. A
laser beam emitted by the semiconductor laser 1a is converted into
a collimated beam by a collimator lens (not shown), and enters a
polarization beam splitter 3 via a cylindrical lens 2a. The
polarization beam splitter 3 has characteristics for transmitting
light coming from the semiconductor laser 1a but reflecting light
coming from the semiconductor laser 1b. Hence, the laser beam
coming from the semiconductor laser 1a is transmitted through the
polarization beam splitter 3 and becomes incident on the rotary
polygonal mirror 4. On the other hand, the laser beam emitted by
the semiconductor laser 1b is converted into a collimated beam by
the collimator lens (not shown), and is then reflected by the
polarization beam splitter 3 via the cylindrical lens 2b. Then, the
laser beam becomes incident on the rotary polygonal mirror 4.
[0044] The laser beams coming from the semiconductor lasers 1a and
1b are reflected by the rotary polygonal mirror 4, are deflected
upon rotation of the rotary polygonal mirror 4, and scan the
surface of the photosensitive drum 6 as a recording medium in the
main scanning direction. Each laser beam reflected by the rotary
polygonal mirror 4 is transmitted through the f-.theta. lens 5
constructed by one or a plurality of lenses, is reflected by the
reflection mirror 9, and then hits the photosensitive drum 6. The
positions of the two laser beams from the semiconductor lasers 1a
and 1b in the main scanning direction are displaced by a
predetermined distance in the sub-scanning direction of the
photosensitive drum 6. The laser beams from the semiconductor
lasers 1a and 1b become incident on the rotary polygonal mirror 4
while being shifted by an angle .theta., and spots of the laser
beams are shifted by a distance f.theta.=.DELTA.A on the surface to
be scanned of the photosensitive drum 6.
[0045] Note that the same applies to a case wherein an integrated
semiconductor laser unit in which two semiconductor lasers emit
laser beams that are initially shifted by the angle .theta., or a
case wherein an integrated semiconductor laser unit having two
semiconductor lasers is used, and the emitted laser beams are
deflected to be shifted by the angle .theta. using a lens and the
like. The positional relationship between the two spots formed by
the two semiconductor lasers 1a and 1b will be described in detail
later.
[0046] In this embodiment, a photodetection element 7 for detecting
the two spots reflected by the rotary polygonal mirror 4 in turn is
placed on the start point side of the two laser beams in the main
scanning direction. The photodetection element 7 comprises two
light-receiving elements which are juxtaposed in the main scanning
direction (or a 2-split photodetection element which is split in
the sub-scanning direction may be used). As these light-receiving
elements, photodetectors are used. The two laser beams reflected by
the rotary polygonal mirror 4 are detected by the two
light-receiving elements of the photodetection element 7 prior to
scanning in the main scanning direction. As will be described in
detail later, the output signals from the two light-receiving
elements are compared by comparison circuits to output a timing
signal indicating a reference position of the two spots. Using this
timing signal, the write start positions of the two spots in the
main scanning direction are synchronized.
[0047] FIG. 4 shows how the two spots formed by the two laser beams
from the semiconductor lasers 1a and 1b pass on the photodetection
element 7 upon rotation of the rotary polygonal mirror 4. Referring
to FIG. 4, spots 8a and 8b are respectively formed by the
semiconductor lasers 1a and 1b. To restate, the photodetection
element 7 is comprised of light-receiving elements 7a and 7b
juxtaposed in the main scanning direction. A spacing D between the
two light-receiving elements 7a and 7b satisfies .DELTA.B>>D
to become sufficiently smaller than the diameter .DELTA.B of each
of the spots 8a and 8b. The two spots 8a and 8b are shifted by
.DELTA.A in the main scanning direction, as described above, and
the spot spacing .DELTA.A in the main scanning direction satisfies
.DELTA.A.gtoreq..DELTA.B to be larger than the spot diameter
.DELTA.B. Furthermore, widths C and E of the two light-receiving
elements 7a and 7b of the photodetection element 7 in the main
scanning direction respectively satisfy C.gtoreq..DELTA.B and
E.gtoreq..DELTA.B to be larger than the spot diameter .DELTA.B.
[0048] The spots 8a and 8b formed by the semiconductor lasers 1a
and 1b are scanned to pass the surface of the photodetection
element 7 in (a), (b), (c), (d), (e), and (f) of FIG. 4. In this
embodiment, when the two spots 8a and 8b pass the surface of the
photodetection element 7, they are turned on in the order from the
leading spot, and the reference timings of the two spots are
detected time-divisionally. This will be described in detail later.
Of the two spots 8a and 8b in FIG. 4, the solid spot indicates ON,
and the broken spot OFF.
[0049] FIG. 5 shows signals of the respective units when the two
spots 8a and 8b pass the surface of the photodetection element 7,
as shown in FIG. 4. FIG. 5 shows an output signal Pa from the
light-receiving element 7a, an output signal Pb from the
light-receiving element 7b, and output signals BDab and Pa' from
comparison circuits. The timings of (a) to (f) in FIG. 5
respectively correspond to (a) to (f) in FIG. 4, and signals at the
timings of (a) to (f) in FIG. 5 indicate those at the two spot
positions in (a) to (f) in FIG. 4.
[0050] FIG. 6 shows an example of the comparison circuits. The
output signals from the light-receiving elements 7a and 7b are
compared by comparators 10 and 11, and their comparison outputs are
output to the controller (printer controller) 126 in the apparatus.
In a comparison circuit 12 shown in FIG. 6, when the output voltage
from the light-receiving element 7a is higher than that from the
light-receiving element 7b, the output signal changes to low level,
and is input to a separation circuit shown in FIG. 7. In a
comparison circuit 13, when the output voltage from the
light-receiving element 7a is higher than a predetermined voltage
Vs, an output signal Pa' changes to low level, and is input to the
separation circuit shown in FIG. 7.
[0051] The operation of this embodiment will be described in detail
below. The operations of the two semiconductor lasers,
light-receiving elements, and comparison circuits will be described
in detail with reference to FIGS. 4, 5, and 6. When the light beams
emitted by the two semiconductor lasers 1a and 1b begin to be
scanned in the main scanning direction, the controller 126 turns on
the semiconductor laser 1a of the leading spot 8a of the two
semiconductor lasers 1a and 1b by setting its drive signal LDa
(shown in FIG. 5) at high level, and turns off the other
semiconductor laser 1b by setting its drive signal LDb (shown in
FIG. 5) at low level.
[0052] In state (a) in FIG. 4, the two spots 8a and 8b begin to be
scanned in the main scanning direction, and are about to reach the
photodetection element 7. In state (a) in FIG. 4, since the leading
spot 8a has not reached the light-receiving element 7a and the
other spot 8b is OFF, the output voltages from both the
light-receiving elements 7a and 7b are 0 level, as indicated by Pa
and Pb in FIG. 5. Also, the output signal from the comparison
circuit 12 is high level, as indicated by BDab in FIG. 5. When the
leading spot 8a has reached the light-receiving element 7a as in
state (b) in FIG. 4, the output signal from the light-receiving
element 7a changes in correspondence with the light amount of the
spot 8a, as indicated by Pa in FIG. 5, and the signal from the
other light-receiving element 7b is 0 level, as indicated by Pb in
FIG. 5, since the spot 8b is OFF. The output from the comparison
circuit 12 is high level, as indicated by BDab in FIG. 5.
[0053] When the leading spot 8a has reached the middle position
between the two light-receiving elements 7a and 7b as in state (c)
in FIG. 4, the output signals from the two light-receiving elements
7a and 7b become equal to each other, as indicated by Pa and Pb in
FIG. 5, and the output signal from the comparison circuit 12 is
inverted from high level to low level, as indicated by BDab in FIG.
5. That is, in this embodiment, the middle position between the
light-receiving elements 7a and 7b is set to be a reference
position, and when the leading spot 8a has reached the middle
reference position between the light-receiving elements 7a and 7b
as in state (c) in FIG. 4, the comparison circuit 12 outputs a
low-level signal indicating that the leading spot has reached the
reference position. This low-level signal is used as a reference
timing signal for the leading spot 8a. When the output signal Pa'
from the comparison circuit 13 changes to high level, the
controller 126 turns off the semiconductor laser 1a of the leading
spot 8a, and turns on the semiconductor laser 1b of the trailing
spot 8b.
[0054] In this state, when the trailing spot 8b has reached the
light-receiving element 7a as in state (d) in FIG. 4, the output
voltage from the light-receiving element 7a changes in
correspondence with the light amount of the spot 8b, as indicated
by Pa in FIG. 5, and the voltage of the light-receiving 7b becomes
0 level, as indicated by Pb in FIG. 5, since the leading spot is
OFF. The output from the comparison circuit 12 is inverted to high
level, as indicated by BDab in FIG. 5. When the trailing spot 8b
has reached the middle position between the light-receiving
elements 8a and 8b as in state (e) in FIG. 4, the output signals
from the two light-receiving elements 7a and 7b become equal to
each other, as indicated by Pa and Pb in FIG. 5, and the output
signal from the comparison circuit 12 is inverted from high level
to low level, as indicated by BDab in FIG. 5. That is, when the
trailing spot 8b has reached the middle reference position between
the light-receiving elements 7a and 7b as in state (e) in FIG. 4,
the comparison circuit 12 outputs a low-level signal indicating
that the trailing spot 8b has reached the reference position. This
low-level signal is similarly used as a reference timing signal for
the trailing spot 8b.
[0055] When the output from the comparison circuit 13 changes to
low level, the controller 126 turns off the semiconductor laser 1b
to end a series of detection processes of the reference timings of
the two spots 8a and 8b in the main scanning direction. In state
(f) in FIG. 4, both the spots 8a and 8b are OFF. In this state, the
output signals from the light-receiving elements 7a and 7b are 0
level, as indicated by Pa and Pb in FIG. 5, and the output from the
comparison circuit 12 is high level, as indicated by BDab in FIG.
5.
[0056] FIG. 7 is a block diagram showing an example of a circuit
for synchronizing the drive and write start timings of the two
semiconductor lasers. The reference position signal Bdab of the two
laser spots is input to a separation circuit (a) 21 in FIG. 7, and
is separated into a reference position signal BDa for the leading
laser 1a and a reference position signal BDb for the trailing laser
1b. For example, the separation circuit is constructed by a
frequency divider, and the like, as shown in FIG. 8, and separates
a signal, as shown in FIG. 9. A time Td shown in FIG. 9 indicates
the operation delay time of a frequency divider 40. The signal BDa
is input to waveform shaping circuits (b) 23 and (c) 32, and the
signal BDb is input to waveform shaping circuits (b) 24 and (d) 34.
For example, the waveform shaping circuit comprises a timer
counter, and outputs a high-level signal a predetermined period of
time after it received a low-level signal.
[0057] FIG. 10 shows the waveforms of the input signals BDa and BDb
and output signals TR-BDa and TR-BDb of the waveform shaping
circuits (a) 23 and (b) 24. Each of the waveform shaping circuits
(a) 23 and (b) 24 sets the signal at high level after an elapse of
a predetermined time Tcount to generate a high-level time Ttr
within a known one main scanning time Tbd. The output signals
TR-BDa and TR-BDb are respectively input to clock synchronous
circuits (a) 25 and (b) 26.
[0058] For example, each of the clock synchronous circuits (a) 25
and (b) 26 comprises M66235FP available from Mitsubishi Electric
Corp., and a reference clock SCLK of an image signal output from an
oscillator 27 is input to the CLKIN terminals of the clock
synchronous circuits (a) 25 and (b) 26. The clock synchronous
circuits (a) 25 and (b) 26 respectively lock the phase of the
reference clock SCLK of an image signal to that of the reference
position signals BDa and BDb, and output locked clocks. In these
circuits, the signals TR-BDa and TR-BDb serve as trigger input
signals, and the phases are locked in response to their trailing
edges.
[0059] FIG. 11 shows a schematic block diagram of M66235FP
available from Mitsubishi Electric Corp. and its input/output
signals. Since the signals TR-BDa and TR-BDb are input to the TR
terminals of the clock synchronous circuits (a) 25 and (b) 26, the
aforementioned time Ttr satisfies Ttr.gtoreq.200 ns. For this
reason, the time Tcount is set at a value that satisfies
Tcount.ltoreq.Tbd-200 ns. The locked clocks CKO (or /CKO) are
respectively input to buffers (a) 28 and (b) 29, which respectively
supply an image signal 30 stored in, e.g., a line buffer to
corresponding laser drive circuits in synchronism with the clocks.
With this image signal, an image is formed.
[0060] FIG. 12 shows the states of the drive signals for the lasers
1a and 1b upon detection of the reference positions. The reference
position signals BDa and BDb input to the waveform shaping circuits
(c) 32 and (d) 34 are converted into drive trigger signals for
drive signals LDa and LDb for turning on the lasers 1a and 1b, and
are input to retriggerable multivibrators (a) 36 and (b) 37. In
these trigger signals, t3 and t4 are set to satisfy Tg<t3 and
t4<Tbd on the basis of a predetermined time Tg required until
the main scan exceeds the image area on the photosensitive drum 6,
and a predetermined one scanning time Tbd. If t3 or
t4.ltoreq.Tbd-200 ns, the waveform shaping circuits (a) 23 and (c)
32 can be constructed by a common circuit, and the waveform shaping
circuits (b) 24 and (d) 34 can also be constructed by a common
circuit.
[0061] The signal Pa' generated based on the output voltage from
the light-receiving element 7a and the threshold voltage Vs is
separated by a separation circuit 22 shown in FIG. 8 into signals
Pa'-1 and Pa'-2, which are respectively input to delay circuits (a)
33 and (b) 35 to be converted into signals Pa'-1' and Pa'-2'
delayed by times t1 and t2. The signals Pa'-1' and Pa'-2' are input
to the retriggerable multivibrators (a) 36 and (b) 37 as reset
signals, i.e., OFF timing trigger signals for the lasers 1a and 1b.
The outputs from the retriggerable multivibrators (a) 36 and (b) 37
become the drive signals LDa and LDb for turning on the lasers 1a
and 1b.
[0062] The delay times t1 and t2 of the delay circuits (a) 33 and
(b) 35 assure the signal duration (low-level time) of the signal
BDab, as shown in FIG. 5, and satisfy t1 or t2.gtoreq.0 (if t1 or
t2=0, the delay circuits are not necessary). The drive signal LDb
for the trailing laser 1b is gated by a gate circuit 38 to be
converted into a drive signal LDb', so that it is turned on only
when the drive signal LDa for the leading laser 1a is OFF, and the
signal LDb' is supplied to a drive circuit 31 for the laser 1b. In
this way, the laser ON timing for detecting the next reference
position is generated. By assuring such delay times, when the
leading spot 8a nearly falls outside the light-receiving element
7a, the laser 1a is turned off.
[0063] In this embodiment, since the two spots are detected by the
two light-receiving elements juxtaposed in the main scanning
direction, and the reference timings of the two spots are detected
on the basis of the output signals from the two light-receiving
elements, no polarization beam splitter is required unlike in the
conventional apparatus, and the arrangement can be simplified. In
an arrangement as shown in FIG. 13 in which a laser spot 151 is
detected by a single light-receiving element 152, and a reference
position is detected on the basis of a prescribed threshold value
Vsc when the laser light amount varies, the output voltage from the
light-receiving element varies like LP1 and LP2 due to different
light amounts, the time until the threshold voltage Vsc is reached
also varies like Ts1 and Ts2, and a time Tj=Ts2-Ts1 is produced.
Hence, when the light amount varies, the time Tj becomes the
deviation component of the reference timing, resulting in poor
positional precision.
[0064] In view of this problem, in this embodiment, even when the
laser light amount varies and the output from one light-receiving
element varies like LP1' and LP2', since the reference timing is
detected by comparing the output signals from the two
light-receiving elements, an accurate reference timing can be
obtained independently of changes in light amount of each of the
two spots. Therefore, the write start positions of the two spots in
the main scanning direction can be accurately aligned, thus
obtaining a high-precision recorded image.
[0065] Furthermore, in this embodiment, the mounting positions of
the two light-receiving elements 7a and 7b need only be adjusted.
Especially, when the two light-receiving elements 7a and 7b are
formed on an IC, since the positional deviation between the two
light-receiving elements remains the same, adjustment can be
greatly simplified as compared to the conventional apparatus. Since
the reference timings of the two spots in the main scanning
direction are detected using one signal line, the controller need
only have one input port.
[0066] Another embodiment of the present invention will be
described below. In this embodiment, as shown in FIG. 15, the width
of the light-receiving element 7b is set to be smaller than that of
the light-receiving element 7a to satisfy C>E. FIG. 15 shows how
the two spots 8a and 8b pass on the surface of the photodetection
element 7 in states (a) to (f), as in FIG. 4. The solid spot
indicates ON, and the broken spot OFF. FIG. 16 shows signals of the
respective units upon passage of the two spots on the surface of
the photodetection element, as shown in FIG. 15. FIG. 16 shows an
output signal (X) from the light-receiving element 7a, an output
signal (Y) from the light-receiving element 7b, and an output
signal (X) from the comparison circuit. The output signal from the
light-receiving element 7b rises earlier since the element 7b has a
smaller area. However, the reference timings of the spots 8a and 8b
in the main scanning direction can be accurately detected as in the
above embodiment. Also, since the area of the light-receiving
element 7b is reduced, the photodetection element can be reduced in
size, and a cost reduction can be attained accordingly.
[0067] In the above embodiments, the two spots 8a and 8b are
simultaneously scanned on the photosensitive drum in the main
scanning direction. Also, the present invention can be used when
three or more spots are simultaneously scanned. For example, when
three spots are simultaneously scanned, a third spot is scanned
after the spot 8b in FIG. 4 to have the same positional
relationship between the spots 8a and 8b. After the reference
timing of the spot 8b is detected, the third spot can be turned on.
In this manner, when the third spot has reached the middle position
between the light-receiving elements 7a and 7b, a reference timing
signal indicating that the third spot has reached the reference
position can be obtained from the output of the comparison
circuit.
[0068] To recapitulate, according to the present invention, not
only the arrangement can be simplified, but also, position
adjustment of the optical elements can be greatly facilitated as
compared to the conventional apparatus. Also, since the reference
timings of the spots are output by comparing the signals from the
two light-receiving elements, even when the amount of the laser
beam that becomes incident on each light-receiving element varies
due to variations in reflectance and contaminations of the
respective reflection surfaces of the rotary polygonal mirror, the
reference timing signal can be accurately output irrespective of
changes in light amount. Therefore, the write start positions of a
plurality of spots in the main scanning direction can be accurately
synchronized, and a high-precision recorded image can be
obtained.
[0069] By "substantially middle position between the two
light-receiving elements" is here meant the middle position of the
distance D where the two light-receiving elements are disposed at
the interval "D" as shown in FIG. 4, or the boundary position
between the two light-receiving elements where the two
light-receiving elements are disposed adjacently each other.
* * * * *